Preheat oven to 350º F.
Bring a large pot of salted water to boil and cook pasta according to packaging directions, or until just al dente. Reserve pasta water.
Wait a few minutes for pasta water to come back up to a boil and add cauliflower florets. Cook for 10-15 minutes, or until fork tender.
Remove cauliflower from pot, shaking off excess water, and transfer to blender or food processor.
Add 2 tablespoons cooking water and pulse until cauliflower is pureed and mostly smooth.
Stir cooked pasta and pureed cauliflower together in a large bowl (or in casserole dish) and stir in milk, sour cream and cheddar cheese.
Season with salt, pepper, garlic powder and nutmeg and mix together.
If not there already, pour mixture into casserole dish and sprinkle grated parmesan cheese evenly over the top.
Place in oven and bake for 20-25 minutes, or until Parmesan is golden and crispy and sauce is bubbly.
Remove from oven and let cool 5 minutes before serving.